Run one frame at a time?

[CleanCut]

Is there a way to run a single frame at a time from a loop external to Bevy? Manually pump the event loop, effectively.

[CleanCut]

@alice-i-cecile answered my question in Discord: Yes! There is even an example of how to do it in examples/app/custom_loop.rs

[CleanCut]

Unfortunately, the custom_loop example runs without a window...and adding default plugins causes the first app.update() to crash with:

thread 'main' panicked at 'Window swapchain node refers to a non-existent window.', /Users/nathan/.cargo/git/checkouts/bevy-f7ffde730c324c74/b6be8a5/crates/bevy_render/src/render_graph/nodes/window_swapchain_node.rs:51:14

This is easy to reproduce by adding .add_plugins(DefaultPlugins) immediately before .set_runner(my_runner) in the custom_loop example.

Does anyone know how to fix this?

Output with `RUST_BACKTRACE=1`

$ RUST_BACKTRACE=1 cargo run --example custom_loop
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/examples/custom_loop`
Type stuff into the console
aosenuth
You typed: aosenuth
thread 'main' panicked at 'Window texture node refers to a non-existent window.', crates/bevy_render/src/render_graph/nodes/window_texture_node.rs:54:14
stack backtrace:
   0: rust_begin_unwind
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/core/src/panicking.rs:92:14
   2: core::option::expect_failed
             at /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/core/src/option.rs:1260:5
   3: core::option::Option<T>::expect
             at /Users/nathan/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:349:21
   4: <bevy_render::render_graph::nodes::window_texture_node::WindowTextureNode as bevy_render::render_graph::node::Node>::update
             at ./crates/bevy_render/src/render_graph/nodes/window_texture_node.rs:52:22
   5: bevy_wgpu::renderer::wgpu_render_graph_executor::WgpuRenderGraphExecutor::execute
             at ./crates/bevy_wgpu/src/renderer/wgpu_render_graph_executor.rs:75:25
   6: bevy_wgpu::wgpu_renderer::WgpuRenderer::run_graph::{{closure}}
             at ./crates/bevy_wgpu/src/wgpu_renderer.rs:115:13
   7: bevy_ecs::world::World::resource_scope
             at ./crates/bevy_ecs/src/world/mod.rs:665:22
   8: bevy_wgpu::wgpu_renderer::WgpuRenderer::run_graph
             at ./crates/bevy_wgpu/src/wgpu_renderer.rs:104:9
   9: bevy_wgpu::wgpu_renderer::WgpuRenderer::update
             at ./crates/bevy_wgpu/src/wgpu_renderer.rs:121:9
  10: bevy_wgpu::get_wgpu_render_system::{{closure}}
             at ./crates/bevy_wgpu/src/lib.rs:112:9
  11: <alloc::boxed::Box<F,A> as core::ops::function::FnMut<Args>>::call_mut
             at /Users/nathan/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1335:9
  12: <bevy_ecs::system::exclusive_system::ExclusiveSystemFn as bevy_ecs::system::exclusive_system::ExclusiveSystem>::run
             at ./crates/bevy_ecs/src/system/exclusive_system.rs:41:9
  13: <bevy_ecs::schedule::stage::SystemStage as bevy_ecs::schedule::stage::Stage>::run
             at ./crates/bevy_ecs/src/schedule/stage.rs:663:21
  14: bevy_ecs::schedule::Schedule::run_once
             at ./crates/bevy_ecs/src/schedule/mod.rs:201:13
  15: <bevy_ecs::schedule::Schedule as bevy_ecs::schedule::stage::Stage>::run
             at ./crates/bevy_ecs/src/schedule/mod.rs:219:21
  16: bevy_app::app::App::update
             at ./crates/bevy_app/src/app.rs:58:9
  17: custom_loop::my_runner
             at ./examples/app/custom_loop.rs:15:9
  18: core::ops::function::Fn::call
             at /Users/nathan/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:70:5
  19: <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call
             at /Users/nathan/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1342:9
  20: bevy_app::app::App::run
             at ./crates/bevy_app/src/app.rs:68:9
  21: bevy_app::app_builder::AppBuilder::run
             at ./crates/bevy_app/src/app_builder.rs:48:9
  22: custom_loop::main
             at ./examples/app/custom_loop.rs:24:5
  23: core::ops::function::FnOnce::call_once
             at /Users/nathan/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
-[_MTLCommandEncoder dealloc]:131: failed assertion `Command encoder released without endEncoding'
[1]    52012 abort      RUST_BACKTRACE=1 cargo run --example custom_loop

[alice-i-cecile]

Can you make an issue for this? Whatever fix there is should be documented in the example itself (or ideally just fixed).

[CleanCut]

Will do 👍🏼

[CleanCut]

Issue created! #1729